Sahara and Sahel vulnerability to climate changes, lessons from Holocene hydrological data

Lézine, Anne-Marie; Hély, Christelle; Grenier, Christophe; Braconnot, Pascale; Krinner, Gerhard

doi:10.1016/j.quascirev.2011.07.006

Cited by 254 publications

(207 citation statements)

References 50 publications

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“…A more noticeable gap between 3ka and 2ka might be more significant although it could also result from under-sampling. Circumstantial evidence supports the possibility that the gap is climatically driven, since it corresponds with a short but clearly defined humid phase between 3.5 and 2.5 ka, identified in the western Sahel by Lézine et al (2011) and overflow from Lake Chad into the Bodélé Depression through the Bahr el Ghazal (Armitage et al submitted), as well as soil development in transverse dunes beside the Bahr el Ghazal (Mauz and Felix-Henningsen 2005).…”

Section: Cumulative Distribution Sar Agesmentioning

confidence: 75%

“…In their review, Lézine et al (2011) omitted dates from sand dunes because they were deemed too scarce to provide a sub-continental record of dry events and too imprecise to be fitted into their mapping intervals. Our results show that, despite the incomplete coverage, it is possible to derive some value from the aeolian record using the limited SAR data available, but that the dune dates do not necessarily correspond with dry events.…”

Section: Discussionmentioning

confidence: 99%

“…Grove 1958, Grove andWarren 1968). We also expected to see progressively younger ages spreading from north to south due to dune reactivation during the late Holocene as the ITCZ moved south (Haug et al 2001), wetlands dried out progressively from north to south (Lézine et al 2011) and the desert expanded southwards again. However, this model does not appear to be supported by the data.…”

Section: Latitudinal Trendsmentioning

confidence: 97%

“…In addition to the latitudinal changes the monsoon system is also believed to have strengthened with the enhanced monsoon spreading further inland and palaeohydrological records suggest that humidity associated with the African Humid period started and ended several millennia earlier to the east than to the west of the Sahara (Lézine et al 2011). Thus changes in dune activity from west to east across the continent might indicate differences in the intensity of the respective monsoons, the WAM in the west and the East African Monsoon (EAM) in the east, as well as the timing of the onset and end of the AHP.…”

Section: Longitudinal Trendsmentioning

confidence: 99%

“…Subsequent papers have questioned the abrupt change (Kröpelin et al, 2008, although the case for an abrupt change was reiterated by McGee et al (2013) who refine the onset and end of the AHP as 11.8 ± 0.2 and 4.9 ± 0.2 ka respectively based upon a review of the marine record. In addition, there is potential for different proxies to respond in different ways andHowever, in their recent review of palynological and palaeohydrological data across the Sahara and Sahel Lézine et al (2011) point out that there will be a lag within wetland response due to groundwater recharge which is likely to have influenced the record of endorehic lake basins as well as ground water fed systems such as Lake Yoa, the source of the gradual change hypothesis. Most recently, Shanahan et al (2015), suggest that the termination of the African humid period was locally abrupt, but occurred progressively later at lower latitudes accompanied by both declining rainfall intensity and gradual southward migration of the tropical rainbelt.…”

Section: Introductionmentioning

confidence: 99%

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Dune ages in the sand deserts of the southern Sahara and Sahel

Bristow

Armitage

2016

Quaternary International

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In this paper we aim to document the history of aeolian processes within the southern Sahara as part of the INQUA Dune Atlas. We review available luminescence ages for sand dunes across the southern Sahara to develop an improved understanding of the dune chronology on a regional basis and attempt to correlate periods of sand accumulation. This was achieved by analysing dune age by country, as well as by latitude and longitude. The results show a very patchy spatial distribution of dune ages with large gaps that encompass some of the largest sand seas. Despite these gaps, some related patterns in dune morphology and stratigraphy appear to be consistent between northern Nigeria and southern Mali where older linear dunes are distinct from younger Late Holocene transverse and barchanoid dunes. Elsewhere in Mauretania linear dunes with different orientations appear to have accumulated at different times, most likely in response to changes in atmospheric circulation. Regional climatic changes are identified where dunes are transgressed by lake deposits within endorehic basins. We identify four locations where dune accumulation is terminated by lacustrine transgressions, two of which, in Lake Chad and the Bodélé Depression, occur shortly after the last glacial maximum (LGM). The third example at Gobiero in Niger occurred later, in the early Holocene, around 8.4 ka and a fourth marks a later transgression of Palaeolake MegaChad after 4.7 ka. Larger-scale latitudinal and longitudinal distributions in dune ages across the southern Sahara do not show any consistent patterns, though this may be due to the small sample size relative to the study area. In addition, local variations in external controls such as wind regime, rainfall, vegetation and sand supply need to be considered, sometimes on a site by site basis. Limiting the analysis to dune ages determined using the single-aliquot regenerative-dose (SAR) protocol indicates a lack of dune preservation during the LGM and the Younger Dryas, times associated with increased dust input to the oceans which is assumed to indicate increased aeolian activity. The SAR dune dates suggest that preservation of dunes at the onset of succeeding humid intervals is an important component of the dune record. The most striking examples of this phenomenon occur where dunes are preserved within endorehic basins by lacustrine transgressions.

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Section: Cumulative Distribution Sar Agesmentioning

confidence: 75%

Section: Discussionmentioning

confidence: 99%

Section: Latitudinal Trendsmentioning

confidence: 97%

Section: Longitudinal Trendsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Dune ages in the sand deserts of the southern Sahara and Sahel

Bristow

Armitage

2016

Quaternary International

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On the remote impacts of mid-Holocene Saharan vegetation on South American hydroclimate: a modelling intercomparison

Tiwari

Ramos

Pausata

et al. 2022

Preprint

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Proxy reconstructions from the mid-Holocene (MH: 6,000 years ago) indicate an intensification of the West African Monsoon and a weakening of the South American Monsoon, primarily resulting from orbitally-driven insolation changes. However, model studies that account for MH orbital configurations and greenhouse gas concentrations can only partially reproduce these changes. Most model studies do not account for the remarkable vegetation changes that occurred during the MH, in particular over the Sahara, precluding realistic simulations of the period. Here, we study precipitation changes over northern Africa and South America using four fully coupled global climate models by accounting for the Saharan greening. Incorporating the Green Sahara amplifies orbitally-driven changes over both regions, and leads to an improvement in proxy-model agreement. Our work highlights the local and remote impacts of vegetation and the importance of considering vegetation changes in the Sahara when studying and modelling global climate.

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The water cycle of the mid‐Holocene West African monsoon: The role of vegetation and dust emission changes

Messori

Gaetani

Zhang

et al. 2018

Intl Journal of Climatology

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During the mid-Holocene (6 kyr BP), West Africa experienced a much stronger and geographically extensive monsoon than in the present day. Changes in orbital forcing, vegetation and dust emissions from the Sahara have been identified as key factors driving this intensification. Here, we analyse how the timing, origin and convergence of moisture fluxes contributing to the monsoonal precipitation change under a range of scenarios: orbital forcing only; orbital and vegetation forcings (Green Sahara); orbital, vegetation and dust forcings (Green Sahara-reduced dust). We further compare our results to a range of reconstructions of mid-Holocene precipitation from palaeoclimate archives. In our simulations, the greening of the Sahara leads to a cyclonic water vapour flux anomaly over North Africa with an anomalous westerly flow bringing large amounts of moisture into the Sahel from the Atlantic Ocean. Changes in atmospheric dust under a vegetated Sahara shift the anomalous moisture advection pattern northwards, increasing both moisture convergence and precipitation recycling over the northern Sahel and Sahara and the associated precipitation during the boreal summer. During this season, under both the Green Sahara and Green Sahara-reduced dust scenarios, local recycling in the Saharan domain exceeds that of the Sahel. This points to local recycling as an important factor modulating vegetationprecipitation feedbacks and the impact of Saharan dust emissions. Our results also show that temperature and evapotranspiration over the Sahara in the mid-Holocene are close to Sahelian pre-industrial values. This suggests that pollen-based paleoclimate reconstructions of precipitation during the Green Sahara period are likely not biased by possible large evapotranspiration changes in the region.

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Sahara and Sahel vulnerability to climate changes, lessons from Holocene hydrological data

Cited by 254 publications

References 50 publications

Dune ages in the sand deserts of the southern Sahara and Sahel

Dune ages in the sand deserts of the southern Sahara and Sahel

On the remote impacts of mid-Holocene Saharan vegetation on South American hydroclimate: a modelling intercomparison

The water cycle of the mid‐Holocene West African monsoon: The role of vegetation and dust emission changes

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